The present invention generally relates to a method and apparatus for producing a magnetic recording medium, and particularly relates to a method and apparatus for producing an elongated belt-like magnetic recording medium such as a magnetic tape or the like.
For producing a magnetic recording medium such as a magnetic tape or the like, there is known a method including steps of continuously conveying an elongated nonmagnetic support in its lengthwise direction, coating the running support with a magnetic coating liquid in which ferromagnetic fine particles are dispersed in a binder using an organic solvent, drying the magnetic coating liquid, and cutting the thus-treated support into a desired shape.
In such a method for producing a magnetic recording medium as described above, the magnetic particles are oriented by means of a magnetic field during the running of the support for the purpose of improving the sensitivity and S/N (signal-to-noise) ratio of the magnetic recording medium. During this process, the magnetic particles (generally, acicular particles) in the magnetic coating liquid applied onto the support are oriented in the conveying direction of the support by means of a magnetic field applied in the support conveying direction before the magnetic coating liquid is dried or while the magnetic coating liquid is being dried so that the rectangular ratio (the ratio of residual flux density to saturation flux density) of the magnetic layer is increased. To produce the magnetic field in the support conveying direction for orienting the magnetic particles in the manner described above, a single or a plurality of electromagnets or permanent magnets are employed.
An example of a conventional magnetic orientation method used in producing magnetic tape is illustrated in FIGS. 5A and 5B, which show a plan view and a side view, respectively, of the region in which orientation is carried out. In this case, the orientation is performed by at least one pair of permanent magnets 5a disposed above and below a support 2 (which has been coated with a magnetic coating liquid) running in the direction of an arrow A in the drawing, with like poles of the two magnets being located opposite to each other with the tape interposed therebetween.
However, the lines x of magnetic force from the permanent magnets 5a are bent outward along the edge portions of the tape (see FIG. 5A), even if the length of the permanent magnets 5a is considerably longer than the width of the support 2. Hence, it is impossible to obtain a magnetic field perfectly parallel to the support conveying direction over the whole width of the support 2. That is, the magnetic particles in the coating liquid are oriented somewhat outward in the vicinity of the edge portions of the support 2 although they can be oriented perfectly in the support conveying direction in the vicinity of the center of the support 2. That is, measurements taken on samples of magnetic tape cut from edge portions of the tape show that the rectangular ratio is lower than that of tape samples cut from the vicinity of the central portion of the support 2. Thus, it is impossible to produce magnetic tapes which are equal in quality over the whole width of the support 2. It is a matter of course that the same problem is caused also in the case of using electromagnets.